Lighting device

ABSTRACT

A lighting device including: a light guide having a first side surface extending in a first direction and a second side surface opposing to the first side surface and extending in the first direction, a first LED being disposed at the first side surface, a second LED being disposed at the second side surface; in which a first area extends in the first direction and a second area extends in the second direction in the light guide; a prism sheet is disposed on the light guide; a prism extending in the first direction and arranged in the second direction is formed on the prism sheet; a first surface of the prism has a first angle with normal direction of the second prism sheet; a second surface of the prism has a second angle with normal direction of the second prism sheet; the first angle is larger than the second angle.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 17/696,965, filed on Mar. 17, 2022, which claims priority fromJapanese Patent Application JP 2021-045714, filed on Mar. 19, 2021, thecontent of each of which is hereby incorporated by reference into thisapplication.

BACKGROUND OF THE INVENTION (1) Field of the Invention

The present invention relates to a lighting device in which emittingdirection can be changed easily or can be collimated in a plurality ofdirections.

(2) Description of the Related Art

Collimated light is sometimes needed in the seats of airplanes ortrains. There are also demands in cars and so forth to emit light indifferent directions according to purposes. On the other hand, there isa demand to emit light in different directions from the light source. Insuch a situation, there are measures as: change a direction of lightsource; providing a reflecting plate to change a direction of light fromthe light source; providing a lens to change a direction of light fromthe light source, and so forth.

Patent document 1 discloses a structure of lighting device, in which arefracting means is set over a direct type light source to change adirection of the light. As for the light refracting means, lenses,prisms, liquid lenses, liquid crystal lenses and so forth are disclosed.

Patent document 2 discloses to use a liquid crystal lens in variousoptical devices.

PRIOR TECHNICAL DOCUMENT Patent Documents

-   Patent document 1: Japanese patent application laid open No.    2012-069409-   Patent document 2: US 2019/0025657 A1

SUMMARY OF THE INVENTION

When collimated light is necessary in each of the seats of airplanes ortrains, or light is necessary in a plurality of directions in e.g. cars,a lighting device can be set in every seats in e.g. an airplane; in thisstructure, however, many lighting devices are necessary, thus, weight orspace for the lighting devices become a problem.

Emitting direction of light can be changed by rotating a light source;however, in this case, a mechanical driving apparatus is necessary,thus, a size of the lighting device becomes large. A method to dispose arefraction means at the emitting surface of light source raises aproblem that the lighting device becomes large due to a refractionmeans, and a driving apparatus for the refraction means is necessary.

The purpose of the present invention is to realize a lighting device ofthin, space saving, and that a direction of light is easily changeable.Another purpose of the present invention is to realize a lighting deviceof thin, space saving, and that collimated lights can be emitted in aplurality of directions.

The present invention solves the above explained problems; examples ofconcrete structures of the present invention are as follows.

(1) A lighting device including: a light guide having a first sidesurface extending in a first direction and a second side surfaceopposing to the first side surface and extending in the first direction,a first LED being disposed at the first side surface, a second LED beingdisposed at the second side surface, in which an area in which the firstLED is disposed and extends in the second direction is defined as afirst area; an area in which the second LED is disposed and extends inthe second direction is defined as a second area; a prism sheet isdisposed on the light guide; a prism extending in the first directionand arranged in the second direction is formed on the prism sheet; afirst surface of the prism has a first angle with respect to normaldirection of the second prism sheet; a second surface of the prism has asecond angle with respect to normal direction of the second prism sheet;and the first angle is larger than the second angle.

(2) A lighting device including: a light guide having a first sidesurface extending in a first direction and a second side surfaceopposing to the first side surface and extending in the first direction,a first LED and a second LED being disposed at either the first sidesurface or the second side surface, in which an area in which the firstLED is disposed and extends in the second direction is defined as afirst area; an area in which the second LED is disposed and extends inthe second direction is defined as a second area; a first prism sheet isdisposed on the light guide; a first prism extending in the firstdirection and arranged in the second direction is formed on the firstprism sheet; light emitted from the first prism sheet is in a normaldirection to the first prism sheet; a second prism sheet, disposed onthe first prism sheet, has a third area in which a second prismextending in the first direction and arranged in the second directioncorresponding to the first area is formed, and a fourth area in which noprism is formed corresponding to the second area; a direction of lightexiting the fourth area is in normal direction of the second prismsheet; and a direction of light exiting the third area forms an anglewith respect to normal direction of the second prism sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a lighting device according toembodiment 1;

FIG. 2 is a detailed perspective view of the area A of FIG. 1 ;

FIG. 3 is a perspective view to show a relation between a light guideand an LED;

FIG. 4 is another perspective view to show a relation between the lightguide and the LED;

FIG. 5 is a cross sectional view in which a prism sheet is disposed onthe light guide;

FIG. 6 includes a detailed plan view, a detailed cross sectional viewand an enlarged cross sectional view of the prism sheet;

FIG. 7 is a cross sectional view to show a function of the prism of theprism sheet;

FIG. 8 is a perspective view which shows a function of embodiment 1;

FIG. 9 is another perspective view which shows a function of embodiment1;

FIG. 10 is yet another perspective view which shows a function ofembodiment 1;

FIG. 11 is a perspective view of embodiment 2;

FIG. 12 is an enlarged perspective view of the area B of FIG. 11 ;

FIG. 13 is a perspective view to show a relation between the light guideand the LED in embodiment 2;

FIG. 14 is a perspective view to show a function when a first prismsheet is disposed on the light guide;

FIG. 15 is a cross sectional view in which the first prism sheet isdisposed on the light guide;

FIG. 16 is a cross sectional view to show a function of the prism of thefirst prism sheet;

FIG. 17 is a perspective view to show a function when a second prismsheet is disposed on the first prism sheet;

FIG. 18 is a cross sectional view to show a function of the prism of thesecond prism sheet;

FIG. 19 is a side view when FIG. 17 is viewed from A direction;

FIG. 20 is a cross sectional view along the line B-B of FIG. 17 ;

FIG. 21 is a cross sectional view along the line C-C of FIG. 17 ;

FIG. 22 is a perspective view to show a first example of embodiment 2;

FIG. 23 is a perspective view to show a second example of embodiment 2;

FIG. 24 is a first example of the second prism sheet; and

FIG. 25 is a second example of the second prism sheet.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is explained in detail according to the followingembodiments.

FIG. 1 is a perspective view of basic structure of a thin lightingdevice according to the present invention. The structure of FIG. 1 is aso called side light configuration in which LEDs 40 are disposed at aside surface of the light guide 10; the side light configuration canmake the lighting device thin. In FIG. 1 , a plurality of LEDs 40 aredisposed in a constant interval at both sides of the light guide 10. TheLEDs 40 set at a right side surface of the light guide 10 and the LEDs40 set at a left side surface of the light guide 10 are disposedalternately to each other with respect to y direction, thus, LEDs 40 donot oppose to each other. Lighting area to the light guide 10 aredifferent in each of the LEDs 40, and the lighting area from the leftside and the lighting area from the right side of the light guide arealternately set in y direction. The LEDs 40 are arranged on the LEDsubstrate 41 in y direction.

A reflection sheet 20 is disposed under the light guide 10. Thereflection sheet 20 reflects the light which goes down from the lightguide 10 to upward to the emitting surface of the light guide 10. Asheet on which aluminum is deposited by vacuum evaporation can be usedas the reflection sheet 20; however, it is preferable to use a sheet onwhich silver, which has higher reflectivity than aluminum, is depositedby vacuum evaporation. For example, ESR (Enhanced Specular Reflector,product of 3M) can be used for the reflection sheet 20; a thickness ofit is approximately e.g. 70 μm. Sometimes, a film of silver evaporatedfilm of a thickness of 0.155 mm is preferable to avoid adhesion betweenthe light guide 10 and the reflection sheet 20.

In FIG. 1 , the light guide 10 is disposed on the reflection sheet 20. Athickness of the light guide 10 is approximately 2 mm. A role of thelight guide 10 is to guide the light from the LEDs 40, incident from theside surfaces of the light guide 10, to upper direction, namely to thelight emitting surface of the light guide 10. However, as will beexplained later, fine prism arrays are formed on the upper surface andon the lower surface of the light guide 10, and the prism arraysdetermine a direction of light from the light guide 10. The lightexiting from the light guide 10 has a distribution with respect to polarangle; the direction in this specification is defined as the directionof main ray, which is a direction of largest light intensity.

The light, which goes down from the light guide 10, is reflected upwardby the reflection sheet 20. The light guide 10, the LEDs 40, thereflection sheet 20, prism sheet 30, and so forth are accommodated inthe outer frame 100.

FIG. 2 is a detailed perspective view of the circle A area of FIG. 1 .The outer frame 100 is omitted in FIG. 2 . The prism arrays are formedon the upper surface and the lower surface of the light guide 10 toguide the light, incident from the side surfaces, to a predetermineddirection from the exiting surface of the light guide 10. Thespecifications of the prism arrays of the upper surface (the majorsurface) and the lower surface are determined as that in what angle thelight is emitted from the major surface of the light guide 10. The belowis examples of prism arrays on the upper surface and the lower surfaceof the light guide 10 according to embodiment 1.

A prism array extending in x direction is formed on the upper surface ofthe light guide 10. The projections of the prism array extend in xdirection and are arranged in y direction. A height of the projection htis e.g. 0.1 μm, a pitch py is e.g. 0.2 μm. An apex angle θt of the primson the upper surface is e.g. 90 degrees.

A prism array extending in y direction and arranged in x direction isformed on the lower surface of the light guide 10. A height of theprojection hb is e.g. 0.002 μm, and a pitch px of the projections ise.g. 0.1 μm on the lower surface of the light guide 10. An apex angle θbof the prims on the lower surface is e.g. 90 degree. In the meantime,the prism arrays formed on the upper surface and on the lower surface ofthe light guide 10 can be formed by forming V grooves on the surfaces ofthe light guide 10 instead of projections.

FIG. 3 is a perspective view of the light guide 10 and LEDs 40 takenfrom FIG. 1 . In FIG. 3 , the LEDs 40 are disposed at both side surfacesof the light guide 10. Each of the LEDs 40 has its own area to supplylight in the light guide 10. The two-dot chain lines defines an area inthe light guide 10 irradiated by each of the LEDs 40. The two-dot chainlines are, however, imaginary lines; there are no such two-dot chainlines in the actual light guide 10. The area 11 is an area whichreceives light from the right side LEDs 40, and the area 12 is an areawhich receives light from the left side LEDs 40. Each of the LEDs 40mainly supplies light in those predetermined areas, however, some lightleaks to other areas.

FIG. 3 shows the LEDs 40 on the left are ON, and the areas indicated bygrey in the light guide 10 are irradiated by LEDs 40 on the left.Therefore, the light is emitted only from grey areas 12 of the lightguide 10 as depicted by the arrow. Herein after, the arrow defines adirection of main ray. As explained in FIG. 2 , prism arrays are formedon the upper surface (main surface) and the lower surface of the lightguide 10; according to those prism arrays, the light exits from theupper surface (main surface) of the light guide 10 in a direction of anangle D1 with respect to the main surface of the light guide 10.

FIG. 4 is another perspective view of the light guide 10 and LEDs 40taken from FIG. 1 . FIG. 4 differs from FIG. 3 in that LEDs 40 on theright are ON, and the areas indicated by grey in the light guide 10 areirradiated by LEDs 40 on the right side. Since each of the prism arraysformed on the upper surface (main surface) and the lower surface of thelight guide 10 are symmetrical, an angle of the exiting light withrespect to the main surface of the light guide 10 is D1, which is thesame value as in FIG. 3 , however, a direction of the exiting light isdifferent from that in FIG. 3 .

FIG. 5 is a side view of the light guide 10, the LEDs 40, the reflectionsheet 20, and the prism sheet 30, which are taken from FIG. 1 . In FIG.5 , the LEDs 40 are disposed at both side surfaces of the light guide10. The LEDs 40, however, are disposed alternatingly in y directionbetween the left side and the right side of the light guide 10. A prismsheet 30 is disposed on the upper surface (main surface) of the lightguide 10. Since the prism sheet 30 has a scalene triangle shape, thedirections of lights which emit from the upper surface (main surface) ofthe light guide 10 with a certain angle are made different between thelight from the LEDs 40 of left hand side and the light from the LEDs 40of the right hand side.

FIG. 6 contains figures of a plan view, a cross sectional view and anenlarged cross sectional view of the prism sheet 30. The prism array inthe prism sheet 30 is formed from V grooves. The prism sheet 30 in FIG.6 is a so called reverse prism sheet. In FIG. 6 , the prism arrayextends in y direction and arranged in x direction. Each of thedimensions in the prism array is as follows. A thickness tp of the prismsheet 30 is e.g. 0.125 mm, a depth vd of the V groove is e.g. 0.075 mm,a pitch pp of the V grooves is e.g. 0.1 mm. Although a cross sectionalview of the general prism sheet 30 is an isosceles triangle with an Apexangle θp of e.g. 68 degree, the cross sectional view in the prism sheet30 in embodiment 1 is different. The cross sectional view of the prismof the prism sheet 30 is, as shown in the bottom figure in FIG. 6 , ascalene triangle.

The prism sheet 30 in embodiment 1 directs the emitting light from thelight guide 10 in different directions between each of the areas. Inother words, the functions of the prism sheet to the light are differentbetween the light emitted from the area 12 of the light guide 10 asdepicted in FIG. 3 and the light emitted from the area 11 of the lightguide 10 as depicted in FIG. 4 . For that purpose, the cross sectionalview of the prism of the prism sheet 30 is made a scalene triangle asshown in FIG. 6 at the bottom.

FIG. 7 is a cross sectional view that shows a function of the prismshown in FIG. 6 . The prism in FIG. 7 is shown as a reverse triangle. Anangle between the first plane 31 and the normal line to the base of thereverse triangle is ell; an angle between the second plane 32 and thenormal line to the base of the reverse triangle is θ12; and θ12>θ11. Thebase of the reverse triangle of FIG. 7 is in a same direction as a mainsurface of the prism sheet 30 and a normal line to the base line of thereverse triangle of FIG. 7 is in a same direction as a normal line tothe prims sheet 30.

The light emitted from the area 12 of the light guide 10 of FIG. 3enters the left surface 32 of the prism of FIG. 7 ; the light refractsat the surface 32, performs a total reflection at the right surface 31,and exits from the upper surface of the prism. On the other hand, thelight emitted from the area 11 of the light guide 10 of FIG. 3 entersthe right surface 31 of the prism of FIG. 7 . Since this light entersthe right surface 31 at a right angle, it goes directly withoutrefraction, performs a total reflection at the left surface 32, refractsat the upper surface of the prism, and exits from the upper surface ofthe prism at an angle D2 with respect to the upper surface of the prism.

As described above, emitting directions can be changed optionally andaccording to the area of the light guide 10 by making a cross section ofthe prism of the prism sheet 30 to a scalene triangle and changing anangle of a prism surface. FIG. 8 is a perspective view that the light isemitted only from the area 12 depicted by grey by only turning on theLEDs 40 of left hand side. In FIG. 8 , the light exiting from the prismsheet 30 is directed to a normal line direction to the prism sheet 30.FIG. 9 is a perspective view that the light is emitted only from thearea 11 depicted by grey by only turning on the LEDs 40 of right handside. In FIG. 9 , the light exiting from the prism sheet 30 is directedto an angle D2 with respect to the major surface of the prism sheet 30.

As described above, the direction of light emitted from the lightingdevice can be changed as that: LEDs 40 of left hand side are turned onto emit light in a normal direction to the lighting device; LEDs 40 ofright hand side are turned on to emit light in a direction ofpredetermined angle from the lighting device. In the meantime, if lightin two directions are needed, the LEDs 40 on the left hand side and theLEDs 40 on the right hand side are turned on simultaneously.

When it is necessary to change a direction of exiting light, the purposecan be performed by only changing the prism sheet 30. Concretely, thelighting device which can change directions of each of two light rays inoptional directions can be realized by changing θ11 and θ12 in FIG. 7 ofthe prism sheet 30.

Embodiment 2

FIG. 11 is a perspective view of the structure of embodiment 2. FIG. 11differs from FIG. 1 in that LEDs 40 as a lighting source are disposedonly on one side surface of the light guide 10, and two prism sheets,the first prism sheet 50 and the second prism sheet 60, are used. Thesecond prism sheet 60 has two areas, namely, the first area 60A, inwhich the prism is formed, and the second area 60B, in which the prismis not formed. Other configurations in FIG. 11 are the same as FIG. 1 .

FIG. 12 is a detailed perspective view of the circle B area of FIG. 11 .The structure of the light guide in FIG. 12 is the same as explained inFIG. 2 of embodiment 1. Disposition pitch of the LEDs 40 in y directionin FIG. 12 is a half of the pitch of the LEDs 40 in y direction in FIG.2 because the LEDs 40 are disposed only on one side surface of the lightguide 10. Accordingly, the structure of the prism sheet is madedifferent from the structure of the prism sheet in embodiment 1;however, the structure of light guide is the same between embodiment 1and embodiment 2.

In FIG. 12 , the first prism sheet 50 is disposed on the light guide 10;the second prism sheet 60 is disposed on the first prism sheet 50. Theprism extends in y direction and a cross sectional view of the prism isan isosceles in the first prism sheet 50. The apex angle of the prism ofthe first prism sheet 50 is set to direct the light emitted from thelight guide 10 in normal direction to the prism sheet 50.

The second prism sheet 60 is disposed on the first prism sheet 50; onlythe first area 60A, in which prism is formed, of the second prism sheet60 is shown in FIG. 12 . The prism array in the second prism sheet 60extends in y direction and is arranged in x direction as the same as theprism array in the first prism sheet 50. The cross sectional view of theprism in the second prism sheet 60 is different from the cross sectionalview of the prism in the first prism sheet 50. The cross sectional viewof the prism in the second prism sheet 60 is rectangular triangle, inwhich only the first side is inclined.

FIG. 13 is a perspective view of the light guide 10 and the LEDs 40,which are taken from FIG. 11 . In FIG. 13 , LEDs 40 are disposed on oneside surface of the light guide 10. The two dot chain lines define anarea that each of the LEDs 40 supplies light. In FIG. 13 , when the LED40 is turned on, the light is emitted from each of the areas of thelight guide 10 in an angle D1 with respect to the major surface of thelight guide 10 according to prism arrays formed on the bottom surfaceand the upper surface of the light guide 10 in each of the areas.

FIG. 14 is a perspective view in which the first prism sheet 50 isdisposed on the light guide 10 of FIG. 13 . The first prism sheet 50directs the light, which is emitted from the light guide 10 in an angleD1 with respect to the upper surface (major surface), to the normaldirection of the first prism sheet 50.

FIG. 15 is a cross sectional view in which arrangement of the lightguide 10, the reflection sheet 20, the first prism sheet 50 and LEDs 40are shown. In FIG. 15 , the prism arrays formed on the upper surface andthe lower surface of the light guide 10 are not shown. The first prismsheet 50 is a so called a reverse prism sheet, in which the prism arrayis formed on the surface opposing to the light guide 10.

FIG. 16 includes a cross sectional view of the first prism sheet 50 anda cross sectional view of the prism in which the function of the prismof the first prism sheet 50 is shown. The prism of the first prism sheet50 is an isosceles triangle. In the cross sectional view of the prism inFIG. 16 , the light, emitted from the light guide 10 at an angle of D1with respect to the major surface, enters the first surface 51 of theprism and goes straight; the light is totally reflected at the secondsurface 52 and goes out in normal direction from the upper surface ofthe prism. The light, emitted from the light guide 10 at an angle of D1in reverse direction with respect to the major surface, enters thesecond surface 52 of the prism and goes straight; the light is totallyreflected at the first surface 51 and goes out in normal direction fromthe upper surface of the prism.

That is to say, the first prism sheet 50 directs the light, emitted ineither the left or the right direction from the light guide 10, tonormal direction of the first prism sheet 50. By the way, the apex angleθ2 of the prism of the first prism sheet 50 is changed according to anexit angle D1 of the light from the light guide 10.

FIG. 17 is a perspective view in which the second prism sheet 60 isdisposed on the first prism sheet 50 shown in FIG. 14 . In the secondprism sheet 60, the first area 60A, in which a prism array is formed,and the second area 60B, in which no prism array is formed, namely, aflat plate, are disposed alternatingly in y direction.

As shown in FIG. 17 , the light emitted from the first area 60A of thesecond prism sheet 60 has a certain angle with respect to the normaldirection of the second prism sheet 60, while the light emitted from thesecond area 60B of the second prism sheet 60 is in normal direction ofthe second prism sheet 60. The light entered in normal direction of thesecond prism sheet 60 from the first prism sheet 50 is changed in thedirection in the first area 60A, while the light goes straight in thesecond area 60B since there is no prism in the second area 60B.

FIG. 18 is a cross sectional view of the prism formed in the first area60A of the second prism sheet 60. The prism in the first area 60A is aso called reverse prism sheet, in which the prism is formed on the lowersurface. The cross sectional view of the prism is a rectangulartriangle; the first surface 61 is in a degree of θ3 with respect tonormal direction to the major surface of the second prism sheet 60; andthe second surface 62 is in normal direction to the major surface of thesecond prism sheet 60.

In FIG. 18 , the light emitted from the first prism sheet 50 is depictedby the arrow. The light emitted from the first prism sheet 50 enters thefirst surface 61 of the first area 60A of the second prism sheet 60.This light refracts at the first surface 61 and refracts again at theupper surface 63 and exits at an angle of D3 from the second prism sheet60. The direction of light exiting from the first area 60A of the secondprism sheet 60 can be controlled by changing the angle of the firstsurface 61 of the prism of the second prism sheet 60.

FIG. 19 is a side view of FIG. 17 viewed from the arrow A. In FIG. 19 ,the reflection sheet 20 is disposed under the light guide 10. In FIG. 19, LEDs 40 are disposed at a side of the light guide 10. Prism arrays areformed on the lower surface and the upper surface of the light guide 10;the light from LEDs 40 incident on the side surface of the light guide10 exits from the upper surface (major surface) of the light guide 10 atcertain angle.

The first prism sheet 50 is disposed on the light guide 10; the lightfrom the light guide 10 is changed to the normal direction to the majorsurface of the first prism sheet 50 by the first prism sheet 50. Thesecond prism sheet 60 is disposed on the first prism sheet 50. The lightentered the first area 60A is refracted by the prism and exits thesecond prism sheet 60 at a predetermined angle. On the other hand, thelight entered the second area 60B of the second prism sheet 60 goesstraight in normal direction. However, the difference in exiting anglesof the light is not perceptible in FIG. 19 .

FIG. 20 is a cross sectional view along the line B-B of FIG. 17 ;namely, the cross sectional view at the first area 60A in which theprism is formed in the second prism sheet 60. The structure of FIG. 20is the same as explained in FIG. 19 . In FIG. 20 , the light emittedfrom the major surface of the light guide 10 enters the first prismsheet 50. This light is changed its direction by the first prism sheet50 to normal direction to the first prism sheet 50, and enters the firstarea 60A of the second prism sheet 60. Then, the light refracts at thefirst surface 61 and at the upper surface 63 of the prism in the firstarea 60A of the second prism sheet 60, and exits the second prism sheet60 at predetermined angle to the normal direction of the second prismsheet 60.

FIG. 21 is a cross sectional view along the line C-C of FIG. 17 ;namely, the cross sectional view at the second area 60B in which theprism is not formed in the second prism sheet 60. The structure of FIG.21 is the same as explained in FIG. 19 . The light pass of the light,emitted from the LEDs 40 and entered from the side surface of the lightguide 10, is the same as explained in FIG. 20 up to exiting the firstprism sheet 50. In FIG. 21 , the light emitted from the first prismsheet 50 enters the second area 60B of the second prism sheet 60, inwhich no prism is formed, consequently, the light goes straight innormal direction to the second prism sheet 60.

FIG. 22 is an example in which only the light of predetermined angle tothe second prism sheet 60 is emitted. In FIG. 22 , the LEDs 40corresponding to the area 60A, depicted as gray, in which prism isformed, are turned on. Since the light is not emitted from the area 60B,only the light at predetermined angle with respect to normal directionof the second prism sheet 60 is emitted.

FIG. 23 is an example in which only the light in normal direction to thesecond prism sheet 60 is emitted. In FIG. 23 , the LEDs 40 correspondingto the area 60B, depicted as gray, in which no prism is formed, areturned on. Since the light is not emitted from the area 60A, only thelight in normal direction to the second prism sheet 60 is emitted.

The second prism sheet 60 can be formed by one sheet as shown in FIG. 24, however, as shown in FIG. 25 , the second prism sheet 60 can be formedby alternatingly arranging the first area 60A and the second area 60B,which are formed separately, on the first prism sheet 50. In themeantime, the second area 60B is not necessary from a view point ofoptical function, however, a simple transparent sheet is used as thesecond area 60B to keep a uniformity in thickness in the second prismsheet 60 since a simple transparent sheet does not have any opticalinfluence.

In the above explanation in embodiment 2, the LEDs 40 are disposed onlyon one side of the light guide 10. The structure of embodiment 2,however, can be applicable to the structure in which the LEDs 40 aredisposed on opposing two sides of the light guide 10. Since the prismarray formed on the light guide 10 is symmetrical, the light emittedfrom the light guide 10, no matter from which sides of the light guide10 the light entered the light guide 10, can be directed to the normaldirection to the first prism sheet 50 by the function of the first prismsheet 50.

As described above, if the LEDs 40 are disposed on both sides of thelight guide 10 in the structure of embodiment 2, the brightness of thelighting device can be improved. In other words, a design freedom ofbrightness in the lighting device can be raised by setting LEDs 40 onboth sides of the light guide 10 in embodiment 2.

When LEDs 40 are disposed on opposing two sides of the light guide 10,the light is emitted other than the intended directions, namely, otherthan the normal direction to the first prism sheet 50 or angle D3direction. If the light having directivity is necessary, it ispreferable not to set the LEDs 40 in opposing relation to each other asthe structure of embodiment 2.

What is claimed is:
 1. A lighting device comprising: a light guidehaving a first side surface extending in a first direction and a secondside surface opposing to the first side surface and extending in thefirst direction, a first LED being disposed at the first side surface, asecond LED being disposed at the second side surface, wherein an area inwhich the first LED is disposed and extends in the second direction isdefined as a first area, an area in which the second LED is disposed andextends in the second direction is defined as a second area, a prismsheet is disposed on the light guide, a prism extending in the firstdirection and arranged in the second direction is formed on the prismsheet, the prism has a first plane extending along the first directionand a second plane extending along the first direction, and istriangular in cross sectional view along the second direction, and afirst angle between the first plane and the normal line to a base of thetriangle is smaller than a second angle between the second plane and thenormal line to the base of the triangle.
 2. The lighting deviceaccording to claim 1, wherein a first prism, which extends in the seconddirection and is arranged in the first direction, is formed on an uppersurface of the light guide, and a second prism, which extends in thefirst direction and is arranged in the second direction, is formed on alower surface of the light guide.
 3. The lighting device according toclaim 2, wherein a height and a pitch of the first prism formed on theupper surface of the light guide is smaller than a height and a pitch ofthe prism formed on the prism sheet, and a height and a pitch of thesecond prism formed on the lower surface of the light guide is smallerthan a height and a pitch of the prism formed on the prism sheet.
 4. Thelighting device according to claim 1, wherein a surface of the prismsheet on which the prism is formed opposes to the light guide.